1. Field of Invention
The present invention relates to a nonvolatile memory device, and more particularly, to a phase change memory device and a fabricating method therefor.
2. Related Art
A phase change memory (PCM) is a nonvolatile memory device. The resistance value of the device can be transformed by changing a crystalline phase of the phase change material through a heating effect.
At present, a chalcogenide phase change material is widely applied in forming the memory cell of the PCM. The chalcogenide is a substance with various solid-state phases, and can be a thereto-induced transition along with a temperature variation. The chalcogenide has a high resistance value when in an anisotropic state (with an irregular atomic arrangement), while has a low resistance value when in a crystalline state (with a regular atomic arrangement). Herein, temperature variations are achieved by providing current or optical pulses, or by another method.
Generally speaking, in a PCM, a transistor is used as a select device to control the current passing through the PCM device and the voltage applied to the chalcogenide. Therefore, in order to reduce the size and power consumption of the PCM, the operation current for the PCM device must be reduced. The current heating effect of the resistance value in the interface region is a function of the contact area of the interface region. Therefore, in the prior art, reducing the operation current may be achieved by reducing the area of the interface region between the current path and the phase change material.
Conventionally, the PCM device is of a T-shape structure, wherein a current path through a phase change layer 130 is formed between the upper and lower electrodes 120, 110, as shown in FIG. 1. A small hole is formed on the dielectric layer 140 through a lithographic process, and then filled with a metal material to form the lower electrode 110, such that the contact area between the lower electrode 110 and the phase change layer 130 is reduced. The contact area between the phase change material (i.e. the phase change layer) and the lower electrode for heating is limited by the capability of the lithographic process. Besides, the small hole is filled with a metal material, easily causing a problem of inadequate step coverage. Moreover, in practice, it is not easy to update the capability of the lithographic process, because equipment must be renewed and personnel must be trained, consuming a great deal of labor and costs.
Therefore, a tapered design is proposed, in which a tip of the tapered lower electrode contacts the phase change layer, thereby reducing the contact area between the two.
Referring to FIG. 2, multiple conductive substrates 111a, 111b, 111c, 111d and a heating electrode 112 are etched in sync to form the tapered structure under an isotropic etching principle. The tapered heating electrode 112 contacts the phase change layer 130, thereby reducing the contact area, as shown in U.S. Pat. No. 6,800,563. However, in practical fabrication with this method, several different materials must he taken into consideration simultaneously when the etching is carried out. Therefore, poor uniformity or undesired etch-pattern problem may occur.
Therefore, an edge contact PCM device has been developed, as shown in FIG. 3. The heating electrode 112 is disposed in the interlayer between the trench sidewalls. The size of the contact area with the phase change layer 130 is controlled by the thickness of the heating electrode 112. However, this may result in difficulty in filling the hole for the phase change material, thus leading to a poor contact of the lateral contact interface and causing a problem in the uniformity and reliability of the device. In addition, the heating electrode extends transversally to contact the phase change material, such that the current path of the heating electrode is increasing. Also, the resistance is high, which causes extra power consumption.
Furthermore, another possible PCM device is a lateral device, as shown in U.S. Pat. No. 6,867,425. Referring to FIG. 4, similarly, the electrodes 114, 122 are disposed in the interlayer between the trench sidewalls, and the size of the contact area with the phase change layer 130 is controlled by the thickness of the electrodes 114, 122. Though the operation current is reduced by the lateral contact, and the path of the current flowing through the phase change material is shortened by controlling the distance between the two electrodes, thereby reducing the power consumption when operating the device, the material of an ordinary heating electrode is usually of a high resistance, and when it serves as a lead, it causes an increase in parasitic resistance, and further causes extra power consumption. Furthermore, when the distance between the two electrodes is relatively small, operational power consumption is reduced, but problems of difficulty, uniformity, and reliability for the phase change material in filling the hole and contacting the sidewall arise.
However, in practice, the restrain of the heat produced by the current also affects desirable reproducibility of the PCM device (i.e., insensitivity to etching damage), resistance aggregation, and low operation current. In a conventional PCM device, when a current is applied to perform Joule heating of the phase change layer, the current and the produced heat are often made to flow and dissipate in a three-dimensional phase change material, so the capability for restraining the heat is rather limited. Also, three-dimensional phase change material is complicated in device characteristic simulation, and it is difficult to achieve accurate simulation and easy authentication.
Also, in the prior arts, in order to reduce the area of interface region between the current path and the phase change material, multiple metal laminations are used. This structure is complicated and is difficult to etch, such that the whole process becomes rather complicated.
Furthermore, in the conventional PCM devices, as the area of the phase change material exposed by the photoresist is excessively large when etching the phase change layer, it leads to a loading effect, thereby producing metal-based polymer residuals at the etched edge of the phase change material. When the heat produced by the current is transmitted to the etched edge of the phase change material, it may cause diffusion, expansion, melting, or reaction of the metal polymer, thereby damaging the device structure.
Similarly, in the conventional PCM device, the heating electrode is mainly fabricated by etching. Therefore, when the area of the heating electrode exposed by the photoresist is excessively large, metal-based polymer residuals arc produced on the etched edge. When the heat produced by the current is transmitted to the etched edge, it may lead to diffusion, expansion, or reaction of the metal-based polymer, thereby damaging the device structure.
Therefore, it is an important development direction for those skilled in the art to provide a PCM device with a low operation current, high reproducibility, and simple process and memory cell structure.